feat: enable pio build cache (#1769)

## Summary

* **What is the goal of this PR?** 
Enables the PlatformIO build cache to speed up compilation.
 
## Additional Context

Significant improvement when switching branches, as unchanged files
won't need re-compilation.
See
[Docs](https://docs.platformio.org/en/latest/projectconf/sections/platformio/options/directory/build_cache_dir.html).

Note: May need to manually delete .cache folder if updating toolchain or
framework.

---

### AI Usage

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Did you use AI tools to help write this code? _**< NO >**_

platformio.ini

@@ -1,5 +1,6 @@
 [platformio]
 default_envs = default
+build_cache_dir = .cache
 extra_configs = platformio.local.ini
 
 [crosspoint]